Various types of systems can be used to separate carbon dioxide from a mixed gas stream. Such systems include pressure-swing absorbers and temperature-swing absorbers. However, these systems are relatively complex, consume high amounts of energy, and are inefficient because of the energy required to rejuvenate the absorber beds as well as the large pressure drops through absorber beds.
Another type of system involves electrochemical reduction and oxidation of carbon dioxide. In such systems an electrochemical cell is used to electrochemically reduce carbon dioxide in one half-cell and electrochemically oxidize carbonate anions in the other half-cell. Although physically viable, neither electrochemical reaction is energetically favorable, requiring platinum-group metal catalysts to proceed. Even with catalysts, the reactions have low energy efficiency. Additionally, the charge carrier, carbonate anions, is somewhat large and results in ohmic losses from transport through an anion exchange membrane.
More recently, electrochemically active organic molecules, such as quinones, have been dissolved in supporting electrolytes and used for oxidation and reduction reactions to separate carbon dioxide. However, the electrolytes are often volatile, flammable, or unstable, and quinone has very limited solubility in the electrolytes—all limiting practical application of quinones.